docs: def get_meta_data(self, request_adu):
          """" Extract MBAP header from request adu and return it. The dict has
          4 keys: transaction_id, protocol_id, length and unit_id.
  
          :param request_adu: A bytearray containing request ADU.
          :return: Dict with meta data of request.
          """
          try:
              transaction_id, protocol_id, length, unit_id = \
                  unpack_mbap(request_adu[:7])
          except struct.error:
              raise ServerDeviceFailureError()
  
          return {
              'transaction_id': transaction_id,
              'protocol_id': protocol_id,
              'length': length,
              'unit_id': unit_id,
          }